Objective: To elucidate the genesis of pathological iron storage in cells and the significance of isoferritins in iron metabolism and cancer. Approach: (1) Investigation of intracellular sites of assembly of ferritin molecules in rat hepatocytes, transplanted rat hepatoma cells and rat hepatoma cells cultured and cloned in vitro, by means of various cell fractionation and radio-immunobiological assays as well as by electron microscopy. Effect of iron on the synthesis and assembly of ferritin in cell components. (2) Analysis of ferritin protein and its subunits produced by several hepatoma cell lines in vitro, using electrophoretic and immunological methods as well as analytical ultracentrifugation. Comparison of the properties of ferritin from highly differentiated hepatomas with those of ferritin from less well differentiated hepatomas and from normal tissues. (3) Analysis of the proteins in bile of iron-loaded rats. Catheterization of common bile duct, collection of bile in a plastic, subcutaneously implanted reservoir. Fractionation of bile, isolation of protein, chemical analysis by several methods.